Bipolar electrolytic cell



April 29, 19 69 J. E. COLMAN BIPOLAR ELECTROLYTIC CELL Filed May 20, 1966 FIG. 3

2? FIG. 4

INVENTOR JOHN E. ZIQMAN FIG. 2

Attorney United States Patent 3,441,495 BIPOLAR ELECTROLYTIC CELL John E. Colman, Oakville, Ontario, Canada, assignor' to Electric Reduction Company of Canada, Ltd., Isllngton, Toronto, Ontario, Canada Filed May 20, 1966, Ser. No. 552,356 Int. Cl. B01k 3/00 US. Cl. 204-268 12 Claims This invention relates to bipolar electrolytic cells. More particularly, this invention relates to bipolar electrolytic cells especially suited to the production of halates, perhalates, or hypohalites of alkali metals, especially chlorates, e.g., sodium chlorate.

Processes of this kind require both a reaction zone and an electrolysis zone, between which the electrolyte can circulate. Taking as an example the production of chlorate, the principal desired reactions taking place in the electrolysis zone are as follows.

Anodic 2c1- c1 +2e 1) CI -I-H OZHOCH-HCI (2) Cathodic 2H++2e H (3) Undesirable side reactions which may take place on the anode within the electrolysis zone are as follows:

In the reaction zone, the following reaction takes place It has been common practice in the past to produce alkali metal chlorates electrolytically by means of a bipolar electrolytic cell positioned in a large container or tank. A typical prior art bipolar electrolytic cell for this purpose consists of a housing in the form of an open-topped box in which a large number of spaced, parallel electrodes, usually of graphite, is positioned. Electrical connections are made to two or more, but not all, of the electrodes for the purpose of supplying electrical energy to the cell. In effect, the electrodes are connected in series electrically through the electrolyte in the cell. At the top and bottom of the housing on both sides thereof are tubes leading into the housing. These tubes constitute inlet (lower) and outlet (upper) tubes and are provided in sufiicient number to communicate with each one of the spaces between pairs of adjacent electrodes. Each pair of adjacent electrodes and the intervening space between them constitute a unit cell. The housing is supported above the floor of the container or tank, and the latter is filled with electrolyte.

The electrolyte enters each unit cell through the lower tubes, these being below the level of the electrolyte in the tank, is electrolysed in the unit cells, and the electrolysed solution is discharged to the tank via the upper tubes, the tank constituting a common reservoir for all unit cells. In many cases, circulation of the electrolyte from the tank to the unit cells and back to the tank occurs without the use of pumps or other such circulating devices, usually because of the production, at one or both of the electrodes, of gas bubbles which begin to rise and tend to carry the electrolyte along with them. However, pumps are sometimes employed. In cases where the tank is positioned remote from the electrolytic cell rather than having the electrolytic cell in the tank, some form of pumping arrangement is usually necessary.

Where sodium chlorate is being produced using a brine electrolyte and graphite electrodes, the electrolytic cell is 3,441,495 Patented Apr. 29, 1969 usually operated at an electrolyte temperature of 20 C. to 45 C. in order to minimize deterioration of the graphite electrodes. In consequence, cooling of the electrolyte in the tank is required in order to maintain temperatures of this order.

One disadvantage of the prior art electrolytic cells relates to the inlet and outlet tubes which provide communication between the housing and the tank. On the one hand, the inlet and outlet tubes must be large enough to permit good circulation of the electrolyte from and to the electrolytic cell when natural circulation is relied upon. On the other hand, the current flowing from one cell to an adjacent cell has some tendency to by-pass any given electrode by following a path through the electrolyte out of one unit cell through a communicating tube to the tank, through which it travels to another communicating tube for a cell further down, which it then enters. This problem, known as current leakage, becomes more and more acute whenever an attempt is made to promote electrolyte circulation by increasing the diameter of the inlet and outlet tubes and/or decreasing their length.

Furthermore, the potential between an electrode in the electrolytic cell and the tank may be quite high, e.g., the end electrodes of the cell may be at +60 and 60 volts respectively, while the tank is at 0 volts. This difference in potential promotes current leakage, the degree of current leakage being greater as the difference in potential increases.

With prior art electrolytic cells of the type hereinbefore described, the maximum voltage drop across the electrolytic cell is usually limited to about volts because of current leakage considerations.

Another disadvantage in the prior art cells is related to control of the pH in the electrolyte. In the production of sodium chlorate by electrolysis it is necessary to add hydrochloric acid to the electrolyte in order to control pH. In the past, this has been done by adding HCl to the reaction zone, the major part of which is the volume of the tank occupied by the electrolyte, as opposed to the volume within the electrolytic cell. Relatively quiet pockets may be formed in the reaction zone of electrolytic apparatus of the aforementioned type, and because of lack of proper mixing, the addition of HCl may cause the pH of the solution in these pockets to fall quite low. This in turn may lead to the evolution of chlorine, which decreases the efiiciency of the electrolytic process. Furthermore, the aforementioned pockets or dead zones decrease the effective reaction zone volume.

In contrast with the above disadvantageous electrolytic cell of the prior art, this invention provides, for use in an electrolytic process requiring a reaction zone and an electrolysis zone, a bipolar electrolytic cell comprising two spaced-apart end electrodes, means for defining with the end electrodes an enclosure adapted to receive and contain an electrolyte, said enclosure being divided intermediate said end electrodes by at least one partition into substantially isolated unit cells. Each partition is constituted at least in part by a bipolar electrode of composite or laminated structure. The composite or laminated structure of the bipolar electrode involves a central or inner plate of a valve metal like titanium or tantalum which could act as a cathode but not an anode, due to the formation under anodic conditions of the oxide of the metal, which oxide has a very high resistance to the passage therethrough of electrons. The oxide itself is chemically resistant under anodic or neutral conditions to the electrolyte to be employed. One face of the central or inner plate is masked by, and in electrically conductive communication with, a layer of a suitable cathodic material chemically resistant under cathodic conditions to the electrolyte to be employed, while a portion only of the other face of the central or inner plate is masked by, and

in electrically conductive communication with, a layer of suitable anodic material chemically resistant under anodic conditions to the electrolyte to be employed. The unmasked portion of the other face is oxidized. The anodic material is so distributed as to cathodically protect the cathodic material of the next adjacent electrode, and is so configured that the unmasked portion of the other face includes at least one vertically elongate area extending over substantially the whole height of the anodic material. For each individual unit cell, there is an individual electrolysis zone and an individual reaction zone substantially isolated from the electrolysis and reaction zones of other unit cells, the electrolysis zone being generally constituted by the electrolyte volume orthogonally (i.e., normally, in the geometric sense) adjacent that one portion, and the reaction zone being constituted at least in part by the electrolyte volume orthogonally adjacent the unmasked portion.

In the immediately preceding paragraph, the term unit cell has been used to refer to one of the chambers or sections into which the electrolytic cell enclosure is divided by the partition, which chambers include both an electrolysis zone and at least part of a reaction zone. This designation is not entirely in accord with the earlier description of the typical graphite bipolar electrolytic cell, where unit cell refers to a volume where, in general, only electrolysis takes place. However, for the sake of terminological clarity, the term unit cell, when used in connection with this invention throughout the remainder of this specification, will refer to a whole one of the chambers or sections mentioned above.

With the construction of this invention as defined, the electrolysis zone (which, in the earlier description of the prior art electrolytic cell, is the unit cell) and at least part of the reaction zone (constituted, in the prior art electrolytic cell, generally by the tank in which the electrolytic cell was immersed) are directly contiguous within a single chamber, referred to in connection with this invention as the unit cell. This construction elimi nates the inlet and outlet tubes which were necessary in the prior art electrolytic cell to connect the individual unit cells with the common reaction zone. This construction also permits good circulation of the electrolyte between the reaction and the electrolysis zones, as will be explained hereinafter.

When a bipolar electrolytic cell according to this invention is operated on a batch or continuous basis, the electrolytic cell can be constructed so that there is virtually no current leakage between the combined reaction and electrolysis zones of one unit cell and the combined reaction and electrolysis zone of an adjacent unit cell, because they are essentially isolated from one another. When adjacent unit cells are cascaded, current leakage can be kept to a minimum by interconnecting the cells in the reaction zone at points reasonably remote from the electrolysis zone, and by making the passages quite small, thereby establishing a path of high electrical resistance. The size of the interconnecting passages can be minimized in the electrolytic cell according to this invention, because their function is no longer essentially to circulate electrolyte between an electrolysis zone and a reaction zone, but merely to pass electrolyte from one reaction zone to an adjacent reaction zone.

Because a bipolar electrolytic cell embodying this invention can be arranged to have essentially no current leakage, voltages across the electrolytic cell in excess of 120 volts can be employed. This is advantageous, since rectifier costs and busbar costs decrease with increasing voltage, so that the cost of rectified current decreases with increasing voltage.

The occurrence of quiet pockets or dead spots in the reaction zones is reduced in the bipolar electrolytic cell according to this invention, due to the improved electrolyte circulation. Thus, it is expected that the effective reaction zone volume should be essentially the whole of the available reaction zone volume, and that chlorine evolution problems due to low pH in quiet pockets and a resultant decrease in efiiciency should be eliminated or minimized.

In an electrolytic cell according to this invention, wherein the anode of the composite bipolar electrode is a platinum metal and the cathode thereof is some suitable metal such as iron, no cooling devices need be used since the electrolyte may be permitted to boil. Under these conditions, the chemical reaction involved in forming chlorate from hypochlorite will proceed about six times as fast as in the prior art apparatus of the type hereinbefore described. Hence, for the same amount of chlorate produced, the reaction zone of an electrolytic cell embodying this invention can be about /6 of that of such prior art electrolytic apparatus.

Further features and advantages of this invention will become more apparent from the following detailed de scription, taken in conjunction with the appended drawings, in which like numbers refer to like parts throughout the several views, and in which:

FIGURE 1 is a plan view of an electrolytic cell embodying this invention;

FIGURE 2 is a sectional view to an enlarged scale taken at the line 22 in FIGURE 1; and

FIGURES 3 and 4 are elevational views of the anodic face of a single bipolar electrode showing variants of the geometric configuration of the platinized layer on the titanium plate.

As used herein, the term bipolar electrolytic cell means an electrolytic cell in which, in use, the electrodes are connected in series electrically, and in which some of the electrodes are bipolar, i.e., one face functions as an anode and the other face functions as a cathode. This is in contrast to a monopolar electrolytic cell in which all of the anodes are connected in parallel, and all of the cathodes are connected in parallel, electrical connections being made between each electrode and the positive or negative terminal of a rectifier.

Referring to FIGURES l and 2, there is shown a bipolar electrolytic cell 10 having a cell tank in the form of a large open-topped box defined by a bottom wall 11, two spaced-apart, parallel side walls 12 and 13 upstanding from the bottom wall 11 at right angles thereto, and two spaced-apart, parallel end walls 14 and 15 also upstanding from bottom wall 11 at right angles thereto and disposed at right angles to the side walls 12 and 13. The tank 10 may be fabricated from any material that is an electrical insulator and that is resistant to chemical attack by the electrolyte at the temperatures to be employed, for example, polyvinyl dichloride. The tank is made liquid-tight.

Although the tank 10 has been shown in the shape of a rectangular parallelepiped, there is no reason why the tank could not assume other shapes.

The electrolytic cell 10 is divided by partitions 16 into substantially isolated unit cells 17. In the embodiment shown, the partitions 16 are constituted wholly by electrodes 18 which are vertically oriented and extend between the side walls 12 and 13 to which they are perpendicularly disposed. It would be possible, however, for a partition 16 to be only partly constituted by an electrode 18, the remainder of the partition being made of some inert material which merely serves to insulate adjacent unit cells from one another.

An alternate method of construction for the electrolyte cell 10 is what is known as the (filter-press type of assembly, where gaskets are interposed between each pair of electrodes 18 (partitions 16) around their peripheries, and pressure is exerted on the end electrodes in order to obtain a compressive liquid-tight seal between the electrodes and the gaskets.

The electrodes 18 are all substantially identical and are equally spaced apart, so that the inter-electrode spaces are of constant width. The electrodes 18, as shown in FIGURES 2 to 4, are rectangular in shape and have flat, parallel faces. The composite or laminated structure of the individual electrodes 18 is best shown in FIG. 2. Each electrode 18 includes a central or inner plate of a valve metal of which the oxide is chemically resistant under anodic conditions to the electrolyte to be employed. The expression valve metal, as used in this specification, is intended to define a metal which can act as a cathode in an electrolytic cell, but not as an anode, due to the formation, under anodic conditions, of the oxide of the metal, which oxide is highly resistant to the passage therethrough of electrons. The preferred valve metal is titanium, although tantalum also is satisfactory. Zirconium and columbium also could be employed. The expression chemically resistant under anodic conditions to the electrolyte to be employed as applied to the oxide, means that the oxide is resistant to the corrosive environment of the electrolyte, and is not, to any great extent, deteriorated, eroded, or attacked. Some metals, such as aluminum, would be unsuitable for this application by reason of forming an oxide layer that would be stripped off in the electrolyte.

One face of the plate 19 is adhered to a layer 20 of suitable cathodic material chemically resistant under cathodic conditions to the electrolyte to be employed. It is not essential, however, that the plate 19 and the layer 20 be bonded directly together, since there is no reason why intermediate layers could not be inserted between the two. Any such intermediate layers need not be in contact with the electrolyte, and it would not be necessary under these circumstances for them to be resistant to attack and corrosion or to be substantially insoluble in the electrolyte under cathodic conditions. It would be necessary, however, that they be capable of forming a good electrically conductive bond with the material on either side. In referring to the layer 20, it could be stated that one face of the plate 19 is masked by, and in electrically conductive communication with, the layer 20, since this will cover the structure whether or not the intermediate layers are present.

The layer 20 is intended to function as the cathode of the bipolar electrode, and the expression suitable cathodic material is intended to refer to a material which is electrically conductive, substantially insoluble in the electrolyte under cathodic conditions, resistant to reduction, and either substantially impermeable with respect to H or if permeable by H dimensionally stable with respect to H Steel is the preferred material, but it would also be possible to use copper, chromium, cobalt, nickel, lead, tin, iron or alloys of the above metals.

One portion of the other face 21 of the plate 19 has bonded thereto a layer of suitable anodic material 22 chemically resistant under anodic conditions to the electrolyte to be employed. The term suitable anodic material" refers to a material that is electrically conductive, resistant to oxidation, and substantially insoluble in the electrolyte. Platinum is the preferred material, but it would also be possible to use ruthenium, rhodium, palladium, osmium, iridium, and alloys of two or more of the above metals. As well, graphite conceivably could be used subject to proper bonding between it and the plate 19, although the concomitant necessity of maintaining the electrolyte at a low temperature makes the use of graphite undesirable. In addition to the above materials, lead dioxide and magnetite also could be utilized as the anodic material.

As with the cathode, it would be possible to interpose any number of intermediate layers between the anodic material 22 and the plate 19, provided that electrically conductive bonding between the layers could be obtained. The expression masked by, and in electrically conductive communication with in the appended claims is intended to cover the construction regardless of Whether or not interposed layers are present. Naturally, if the interposed layers contact the electrolyte at any point, either (a) they would have to have the same characteristics as the anodic material, or (b) they would have to be valve metals capable of forming an impervious oxide layer proof against attack by the electrolyte.

The anodic material should be generally distributed over the face 21 of the plate 19 for two main reasons. Firstly, a general distribution of the anodic material will help to protect cathodically the cathodic material of the adjacent electrode. Secondly, it will not leave a large area of the cathodic material without some anodic material generally across from it, in which area there would have been a danger of bad circulation and stagnation, with the resultant lowering of efiiciency.

In FIG. 3, the platinized area 22 is roughly centered on the titanium surface 21, and a configuration of this kind would be satisfactory provided proper circulation were obtained in the unplatinized area remote from the platinized area. Because of this and other considerations, it might become necessary to split the anodic area 22 into two or more horizontally adjacent, non-contiguous sections 24 and 25 (FIG, 4) distributed across the face 21. The combined area of the sections 24 and 25 is the rough equivalent of the area of the section 22.

It is considered essential that the portion of face 21 masked by the anodic material be so configured that the unmasked and therefore oxidized portion of the face 21 include at least one vertically elongate area extending over substantially the whole height of the anodic material. As can be seen, the configurations shown in FIGS. 3 and 4 answer to this description. Another possible configuration which would include the abovementioned vertically elongate unmasked area extending over substantially the whole height of the anodic material would be one in which the platinized portions extended inwardly from the two lateral edges of the face 21 and defined between them a vertical area or channel somewhat similar to that shOWn between the sections 24 and 25 in FIG. 4. A configuration which would be unsuitable, however, would be one in which the platinized areas consisted of horizontal bands running completely across the face 21 from one lateral edge to the other.

It is not essential that the platinized portions extend all the way from the top to the bottom of the face 21, particularly in view of the fact that, in most cases, the electrolyte surface will be located at a point lower than the top of the face 21. As can be seen, FIG. 3 shows the platinized area terminating short of the upper and lower edges of the face 21, whereas FIG. 4 shows the platinized areas 24 and 25 extending the whole height of the face 21.

The reason for the requirement of the vertically elongate area extending over substantially the whole height of the anodic material relates to the promotion of proper circulation of the electrolyte within the unit cell Before discussing the electrolyte circulation in greater detail, it will be useful to set down convenient definitions for the electrolysis zone and for the reaction zone for each unit cell. The electrolysis zone is considered here to be the electrolyte volume orthogonally adjacent the portion of the face 21 which is masked by the anodic material. In other words, the electrolysis zone is the envelope of all the points in the electrolyte from which a normal line can be drawn to the anodic material-hence the expression orthogonally adjacent. This definition is permissible because substantially all of the electrolysis takes place within this zone as defined, although due to the divergence of the current path from the anodic areas to the cathode, some of the cathodic electrolysis will take place outside of the defined zone. The reaction zone is defined as being constituted at least in part by the electrolyte volume orthogonally adjacent the unmasked portion of the face 18. There are two reasons for the expression at least in part in the latter definition. Firstly, it is not considered necessary that the whole of the reaction zone be contained within the unit cell 17, although this is the preferred arrangement. It also would be possible to provide, for each individual unit cell, one or more supplementary reaction zones outside the unit cell 17 but in fluid communication with the interior reaction zone. Secondly, whereas substantially all of the electrolytic reaction of a given unit cell takes place within the electrolysls zone for that cell as above defined (omitting from consideration the slight cathodic electrolysis taking place outside of the defined electrolysis zone), the reaction equations do not require an electrode surface and take place generally throughout the electrolyte volume, including the electrolysis zone. However, the major portion of the reaction will take place in the reaction zone as defined above.

Returning now to the problem of electrolyte circulation, the electrolytic processes to which this invention is directed generally involve the production of minute gas bubbles at the cathodic surface when electrolysis is there taking place. These gas bubbles immediately begin to rise, and tend to promote upward flow in the electrolyte surrounding them, By configuring the anodic material on the face 21 such that the unmasked portion includes at least one vertically elongate area, the electrolyte which is urged upwardly by the gas bubbles has a kind of return channel where it can flow back downwardly in the unit cell. Thus, a vigorous circulation is promoted, good mixing is attained, and both the chemical and electrolytic reactions can proceed at maximum efiiciency.

In this connection, a particular advantage is attendant upon the configuration shown in FIG. 4. This is the high degree of mixing which takes place in the vertical channel between the platinized sections 24 and 25. Electrolyte flowing upwardly across the faces of the platinized portions 24 and 25 will branch either way at the electrolyte surface near the upper edge of the face 21, such that roughly half of each of these upward currents will descend along the vertical channel between the sections. Thus, the two separate stream are mixed together in this channel.

It is preferable that the portion of the face 21 which is masked by the anodic material have an area which is between 10% and 80% of the total area of the face 21, although these figures are not considered to be critical. What is important is that the anodic, massed portion be large enough to function efficiently as an anode, so distributed as to cathodically protect substantially all of the cathodic surface of the next adjacent electrode, and configured in such a way that the unmasked area includes, as aforementioned, at least one vertically elongate area extending over substantially the whole height of the anodic material.

By substituting certain other metals or graphite for iron, and by employing a diaphragm, the cell described herein can be modified for use in the production of caustic chlorine.

Naturally, the compound bipolar electrolytic cell according to this invention can be used for both batch operation and cascade operation. When used for batch operation, the individual unit cells are completely isolated from one another, and each unit cell is provided with an inlet tube and an outlet tube for, respectively, charging the unit cell with electrolyte and removing the electrolyte therefrom. When the individual unit cells of a bipolar electrolyticcell according to this invention are cascaded, apertures or tubes through the electrodes are used to interconnect adjacent unit cells, and permit passage from one cell to another of the electrolyte. For cascade operation, it is advisable that the apertures or tubes used to interconnect adjacent unit cells be positioned as remotely as possible from the areas masked by the anodic material, due to current leakage considerations.

While a preferred embodiment of this invention has been disclosed herein, those skilled in the art will appreciate that changes and modifications may be made therein without departing from the spirit and scope of this invention as defined in the appended claims.

What I claim in my invention is:

1. For use in an electrolytic process requiring a reaction zone and an electrolysis zone, a bipolar electrolytic cell comprising two spaced-apart end electrodes, means for defining with said end electrodes an enclosure adapted to receive and contain an electrolyte, said enclosure be ing divided intermediate said end electrodes by at least one partition into substantially isolated unit cells, each partition being constituted at least in part by a bipolar electrode comprising a plate of a valve metal of which the oxide is chemically resistant under anodic conditions to the electrolyte to be employed, one face of the plate being masked by, and in electrically conductive communication with, a layer of suitable cathodic material chemically resistant under cathodic conditions to the electrolyte to be employed, one portion of the other face of the plate being masked by, and in electrically conductive communication with, a layer of suitable anodic material chemically resistant under anodic conditions to the electrolyte to be employed, the unmasked portion of the other face being oxidized, the anodic material being so distributed as to protect cathodically the cathodic material of the next adjacent electrode, and being so configured that the unmasked portion of said other face includes at least one vertically elongate area extending over substantially the whole height of the anodic material, there being for each individual unit cell an individual electrolysis zone and an individual reaction zone, substantially isolated from the electrolysis and reaction zones of other unit cells, the electrolysis zone being generally constituted by the electrolyte volume orthogonally adjacent said one portion, and the reaction zone being constituted at least in part by the electrolyte volume orthogonally adjacent said unmasked portion.

2. An electrolytic cell as claimed in claim 1, in which the whole of the individual reaction zone for each individual unit cell lies within the electrolyte volume orthogonally adjacent the said plate.

3. An electrolytic cell as claimed in claim 1, in which -the faces of the plate are substantially planar and parallel.

4. An electrolytic cell as claimed in claim 1, in which the anodic material is platinum, ruthenium, rhodium, palladium, osmium, iridium, or alloys of two or more of the foregoing metals.

5. An electrolytic cell as claimed in claim 1, in which the valve metal is titanium, tantalum, zirconium, or columbium.

6. An electrolytic cell as claimed in claim 1, in which the cathodic material is steel, copper, chromium, cobalt, nickel, lead, tin, iron, or alloys of the foregoing metals.

7. An electrolytic cell as claimed in claim 1, in which the area of said one portion is between 10% and of the total area of said other face.

'8. An electrolytic cell as claimed in claim 7, in which zaid one portion is substantially centered on said other ace.

9. An electrolytic cell as claimed in claim 7, in which said one portion consists of a plurality of horizontally adjacent non-contiguous sections distributed across said other face.

10. An electrolytic cell as claimed in claim 1, in which the bipolar electrode constitutes the whole of the partition.

11. An electrolytic cell as claimed in claim 10, in which the whole of the individual reaction Zone for each individual unit cell lies within the electrolyte volume orthogonally adjacent the said plate.

12. An electrolytic cell as claimed in claim 11, in which (a) the anodic material is bonded directly to the plate and is platinum, ruthenium, rhodium, palladium, osmium, iridium, or alloys of two or more of these metals;

9 10 (b) the valve metal is titanium, tantalum, zirconium, or 968,492 8/1910 McDorman 204268 3,350,286 10/1967 Crane 204-95 columbium; and (c) the cathodic material is bonded directly to the plate and is steel, cooper, chromium, co-

balt, nickel, lead, tin, iron, or alloys of these metals. HQWARD S-WILLIAMS, Primary Examiner- 5 D. R. JORDAN, Assistant Examiner.

References Cited UNITED STATES PATENTS US. 01. X.R. 685,274 10/1901 Haas 204-268 204-290 

1. FOR USE IN AN ELECTROLYTIC PROCESS REQUIRING A REACTION ZONE AND AN ELECTROLYSIS ZONE, A BIPOLAR ELECTROLYTIC CELL COMPRISING TWO SPACED-APART END ELECTRODES, MEANS FOR DEFINING WITH SAID END ELECTRODES AN ENCLOSURE ADAPTED TO RECEIVE AND CONTAIN AN ELECTROLYTE, SAID ENCLOSURE BEING DIVIDED INTERMEDIATE SAID END ELECTRODES BY AT LEAST ONE PARTITION INTO SUBSTANTIALLY ISOLATED UNIT CELLS, EACH PARTITION BEING CONSTITUTED AT LEST IN PART BY A BIPOLAR ELECTRODE COMPRISING A PLATE OF A VALVE METAL OF WHICH THE OXIDE IS CHEMICALLY RESISTANT UNDER ANODIC CONDITIONS TO THE ELECTROLYTE TO BE EMPLOYED, ONE FACE OF THE PLATE BEING MASKED BY, AND IN ELECTRICALLY CONDUCTIVE COMMUNICATION WITH, A LAYER OF SUITABLE CATHODIC MATERIAL CHEMICALLY RESISTANT UNDER CATHODIC CONDITIONS TO THE ELECTROLYTE TO BE EMPLOYED, ONE PORTION OF THE OTHER FACE OF THE PLATE BEING MASKED BY, AND IN ELECTRICALLY CONDUCTIVE COMMUNICATION WITH A LAYER OF SUITABLE ANODIC 